1. Field of the Invention
The present invention relates to an automatic gain control (AGC) circuit, and more particularly, to an AGC circuit suitable for a television receiver, a video tape recorder or a video disc player.
2. Description of the Prior Art
FIG. 1 is a block diagram showing an example of a conventional AGC circuit. An input picture signal 100a is amplified by an amplifier 1 and resulted to an output picture signal 100b with an AGC as described in detail later. Output picture signal 100b is not only introduced to an output terminal 8, but also applied to a clamping circuit 2. In clamping circuit 2, a tip-end level of a synchronous signal component of the output picture signal 100b is clamped to a predetermined fixed voltage and resulted to a tip-end clamped picture signal 200. Clamped picture signal 200 outputted from clamping circuit 2 is applied to a synchronous detection circuit 3 and a peak detection circuit 4, respectively. Synchronous detection circuit 3 detects a pedestal level of the clamped picture signal 200 by a synchronous or keyed detection in the reference of the tip-end level of the synchronous signal component under a synchronization of a verst signal. While peak detection circuit 4 detects a peak level of the picture signal by a peak detection in the reference of the tip-end level of the synchronous signal component. First level signal 300a thus detected by synchronous detection circuit 3 is smoothed by a first filter 5 and resulted to a first AGC signal 400a. First AGC signal 400a is applied to a control terminal of amplifier 1 for controlling the gain of amplifier 1. As a result, output picture signal 100b with synchronous detection AGC is obtained an output terminal 8 as mentioned before. In the synchronous detection AGC, an amplitude of the synchronous signal component in the picture signal 100a is controlled so as to keep in constant.
On the other hand, second level signal 300b thus detected by peak detection circuit 4 is also smoothed by a second filter 6 and resulted to a second AGC signal 400b. Second AGC signal 400b is applied to the control terminal of amplifier 1 through a buffer 7 for controlling the gain of amplifier 1. As a result, output picture signal 100b with peak detection AGC is obtained an output terminal 8 as also mentioned before. In the peak detection AGC, an amplitude of the picture signal 100a is controlled so as not to exceed a predetermined amplitude.
In the AGC circuits, synchronous detection circuit 3 makes a synchronous detection AGC loop together with amplifier 1, clamping circuit 2, and first filter 5. While peak detection circuit 4 makes a peak detection AGC loop together with amplifier 1, clamping circuit 2, second filter 6 and buffer 7.
In the example of the conventional AGC circuits as described above, the synchronous detection AGC loop operates so as to hold an amplitude of first level signal 300a in a given level so that output picture signal 100b is held its amplitude constant. However, there may happen a problem that the amplitude level of output picture signal 100b exceeds the given level relatively. For overcoming the problem, the peak detection AGC loop operates so as to limit the amplitude level of output picture signal 100b to the given amplitude level. Accordingly, the amplitude level of output picture signal 100b on output terminal 8 is so controlled that its amplitude is limited within the given level, as well as an amplitude of its synchronous signal component is held constant.
By the way, when incorporating the conventional AGC circuit described above in a video tape recorder, the AGC circuit operates for preventing an overmodulation in the video tape recorder. For the overmodulation preventing operation, the AGC circuit is required to have a prompt response. However, in the conventional cirucit shown as in FIG. 1, the AGC circuit fails to prevent promptly the overmodulation. Because second filter 6 has been discharged in 0 volt state during the time when the peak detection AGC loop is deactivated. Therefore, the peak detection AGC loop fails to control immediately amplifier 1 into the peak detection AGC operation state after the peak detection AGC loop has been activated or started. The defect of slow response is extremely inconvenient to the function of preventing the overmodulation described above.